Method for forging a titanium alloy thermomechanical part

ABSTRACT

A method for forging a thermomechanical part and including: providing a billet produced in a titanium alloy having a beta transus temperature; carrying out at least one operation of forging a blank of the billet at a temperature T 1  lower than the beta transus temperature T b  from before carrying out the forging operation whereby a blank is completed; carrying out a final forging the blank at a temperature T 2  greater than the beta transus temperature T b  from before carrying out the forging operation whereby a blank is completed. The forging operation from the blank-forging carries out, on every point of the billet, a deformation greater than a minimum deformation rate. The method can be used for a rotating part of a turbine engine.

The invention relates to a method of forging a thermomechanical part made of an alloy of beta or alpha/beta titanium.

The invention also relates to a method of fabricating a thermomechanical part, the method including the forging method.

The invention also relates to a thermomechanical part resulting from the forging method or from the fabrication method, said thermomechanical part being a beta-forged alpha/beta alloy forging presenting a microstructure that is fine and uniform with a grain size of the order of 50 micrometers (μm) to 100 μm.

The invention also relates to a turbomachine including such a thermomechanical part.

Most particularly, but in non-limiting manner, the invention applies to the rotary parts of turbomachines such as the disks, trunnions, and impellers, and in particular to high-pressure compressor disks, in particular integrally bladed rotors (IBRs). Such rotary parts typically present a thickness greater than 10 millimeters (mm), or even 20 mm or 30 mm.

The present invention relates to all types of temperature-stabilized titanium alloy: titanium alloys of the beta class and of the alpha/beta class (where these terms refer to the structure of the finished part).

The present invention relates more particularly to the titanium alloys known as “beta-forged alpha/beta” alloys, where the mention “alpha/beta” corresponds to the microstructure of the part, i.e. to the coexistence of the alpha and beta phases of titanium, the part being shaped by forging. The forging method includes in particular a final step of deforming the titanium alloy into the beta domain by stamping.

It is recalled that the beta domain of the titanium alloy corresponds to temperatures higher than the beta transus temperature T_(β), where temperatures lower than the beta transus temperature T_(β) correspond to the alpha/beta domain.

At present, in the technique used by the Applicant to fabricate high-pressure compressor disks, including IBRs, the forging method corresponds to the diagram of FIG. 1, described below.

Initially, a titanium alloy ingot obtained by casting is transformed into a billet presenting any desired shape, which is usually a cylindrical shape.

Such a billet constitutes a semi-finished product and it is obtained by melting the master alloy one or more times and then casting an ingot that is itself forged in application of a precise thermomechanical cycle (which does not correspond to the forging method of the present invention), with this being done for the purposes of reducing the section of the ingot and of obtaining the billet with controlled metallurgical and dimensional characteristics.

By way of example, the melting operation(s) is/are performed using one of the following techniques: vacuum arc remelting (VAR), electron beam cold hearth refining (EBCHR), or plasma arc melting (PAM).

The billet is then subjected to the forging method illustrated in FIG. 1 by a plot of the temperature to which the billet is subjected as a function of time.

As a general rule but not always, a first forging step is initially performed that consists in one or more intermediate forging operations or “blank forging”.

During such blank forging, the billet is initially heated (reference a) between times t₀ and t₁ from ambient temperature T₀ up to a temperature T₁ that is lower than the beta transus temperature T_(β). Usually, this temperature T₁ is of the order of the beta transus temperature minus sixty degrees (T_(β)−60° C.), and this temperature rise, that depends on the bulk of the billet, takes about 2 hours (h) for a billet having a diameter of 200 mm, for example.

Thereafter, the billet is maintained at the temperature T₁ (reference b) between times t₁ and t₂, corresponding to a duration of about 1 h, or more, in order to ensure that all of the material constituting the billet has reached the temperature T₁, prior to proceeding with the forging operation proper (reference c), i.e. hot plastic deformation performed by means of a press (stamping), hammer, rolling mill, . . . and applied to the billet during times t₂ and t₃, corresponding to a duration of a few tens of seconds, thereby forming a blank. During this forging operation, the blank is in ambient air, so the surface of the part naturally cools through a few tens of ° C. while the core of the part cools a little or even heats up by a few ° C., depending on the bulk of the part and on forging conditions, and in particular on the rate of deformation.

Finally, in order to finish the forging blank, the blank is allowed to cool (reference d) down to ambient temperature T₀, between times t₃ and t₄, corresponding to a duration of a few tens of minutes, approximately.

From time t₄, either the blank is left at ambient temperature T₀ until a time t_(n) at which the second forging step or final forging begins, or else second or additional other blank forging operations (references a′, b′, c′, d′ for a second blank forging) are performed that is/are similar to the first blank forging (references a, b, c, d) as described above. Thus, when a second or additional other blank forging operations are performed prior to performing the second or final forging step, the forging operation proper is always performed at a temperature T₁ lower than the beta transus temperature T_(β), and in particular at the same temperature T₁ as was used for the first blank forging.

Under such circumstances, an alternative consists in beginning a second blank forging operation sooner by reheating the blank (reference e) between times t₃ and t₄ of the first blank forging operation, i.e. without waiting for the blank to cool down completely to ambient temperature T₀ (reference d for the first blank forging). Under such circumstances, the second blank forging operation is begun by repeating the temperature rise of the blank (reference e) up to the temperature T₁, and then continuing by maintaining the temperature (reference b′) prior to the forging operation proper (reference c′). This alternative serves to reduce the time taken by the forging method without running the risk of causing the microstructure of the billet to vary during complete cooling and subsequent temperature rise (references d and a′).

For the second or final forging step, which begins at time t_(n), the steps performed are similar to those of the blank forging operation, except for the value of the temperature to which the blank is raised prior to performing the forging operation proper, since the temperature is now a temperature T₂ that is higher than the beta transus temperature T_(β). Conventionally, this temperature T₂ is of the order of the beta transus temperature plus twenty-five degrees (T_(β)+25° C.).

More precisely, the final forging comprises heating the blank (reference A) between times t_(n) and t_(n+1) from ambient temperature T₀ up to the temperature T₂, and then maintaining it at the temperature T₂ (reference B) between times t_(n+1) and t_(n+2), prior to performing the forging operation proper (reference C) on the blank between times t_(n+2) and t_(n+3). This operation of forging the blank (reference C) is performed at the temperature T₂ in the beta domain (temperature higher than T_(β)), with the progressive cooling of the blank during this forging operation possibly leading to a portion of the blank that is subjected to the forging operation presenting a temperature that is lower than T_(β) and thus also being forged at a temperature that corresponds to the alpha/beta domain. Finally, the forging as obtained in this way is cooled (reference D), with this forged blank or forging being cooled to ambient temperature T₀ between times t_(n+3) and t_(n+4).

The other forging parameters of the blank forging steps and of the final forging step, and in particular the rate of forging, the time for transfer between the heating furnace and the forging equipment, the time for transfer between the forging equipment and the system for cooling the part after forging, are defined as a function of the shape and the bulk of the forging and also as a function of the available industrial equipment.

The number of blank forging operations and also the characteristics of each forging operation proper (references c, c′, . . . , C) in the blank forging step and in the final forging step, and in particular the forging equipment selected (hydraulic press, mechanical screw press, hammer, rolling mill), the position of the billet/blank relative to the forging tool, the level of stress that is exerted and its duration, and also the number of repeats are all defined for each type of part, depending on its shape and bulk, in application of a pre-established procedure that enables the billet and then the blank to be deformed progressively so as to form, at the end of the forging method, a forging that presents the required geometrical characteristics.

During each forging operation proper (references c, c′, . . . , C) of the blank forging step and of the final forging step, the part is subjected to deformation that is both of macroscopic and of microscopic order.

At the end of the final forging operation, a forging is obtained that forms a product that can be said to be a finished product in the sense that it is no longer subjected to subsequent forging operations and/or plastic deformation operations; this product is subsequently machined and subjected to additional treatment, in particular surface conditioning as a function of its conditions of use, in particular within the engine forming the turbomachine.

That prior art method of fabricating a forging is usually satisfactory. Nevertheless, under certain circumstances, there is a risk of forming a forging that does not comply correctly with all of the criteria for guaranteeing its expected mechanical properties.

In spite of all of the precautions that are taken during its development, it can sometimes happen that the billet of titanium alloy that is subjected to the above-described method of fabrication by forging, initially presents a microstructure that is not uniform but is heterogeneous. In particular, it is possible to encounter a microstructure that contains one or more large grains of titanium, possibly presenting a size of as much as several millimeters, or even of centimeter order, in particular grains of beta phase titanium. These large grains that have not recrystallized into smaller grains form isolated islands that, because of their large size, are not refined, i.e. they are not transformed into recrystallized grains of smaller size by the above-described forging method.

This situation occurs most particularly as a result of the large size of the parts in question, in particular their significant height, which may be of the order of 100 mm to 200 mm, or even as much as 250 mm, such that the starting billets (or slugs) themselves present large dimensions, e.g. a diameter of the order of 250 mm.

An object of the present invention is to provide a forging method that enables the drawbacks of the prior art to be overcome, and in particular that makes it possible to cause the presence of any non-uniform microstructure in the blank to disappear, and in particular to cause the presence of any large grains in the starting billet to disappear so as to provide the forging with a microstructure that is uniform.

To this end, the present invention relates to a method of forging a thermomechanical part of beta or alpha/beta titanium alloy, the method comprising the following steps:

-   -   providing a billet made from a titanium alloy possessing a beta         transus temperature T_(β);     -   performing at least one blank forging step on said billet, in         which said billet is heated to a temperature T₁ lower than the         beta transus temperature T_(β) prior to performing the forging         operation proper during which said billet is subjected to         plastic deformation, thereby obtaining a blank, and then         allowing the blank to cool; and     -   performing a final forging step on said blank, in which said         blank is heated to a temperature T₂ higher than the beta transus         temperature T_(β) prior to performing the forging operation         proper during which said blank is subjected to plastic         deformation, thereby obtaining a forging, and then cooling said         forging.

According to the invention, the method is characterized in that said forging operation of the blank forging step implements, at all points of said billet, local deformation that is greater than a minimum deformation ratio.

The term “deformation ratio” is used herein to mean the accumulated plastic deformation at a point of the part, also known as “equivalent deformation” with this being taken into consideration on the part that has been subjected to the blank forging operation under consideration.

The idea is thus to perform a forging operation during the blank forging step (or at least during one of the blank forging steps if there is more than one of them), such that some minimum amount of local deformation is achieved at all points in the billet, i.e. the billet is subjected not only to overall deformation, but above all to some minimum amount of local deformation at all points.

Thus, the solution of the present invention amounts to modifying the deformation conditions imposed on the billet during the forging method at the time of the forging operation proper (reference c and/or c′) in at least one of the blank forging steps, i.e. for the forging operation(s) performed in the alpha/beta domain, i.e. below the beta transus temperature T_(β).

It should be observed firstly that the solution of the invention applies during the blank forging step and not during the final forging step, and secondly that the solution of the invention relies on ensuring that some minimum amount of deformation occurs locally and not on ensuring some minimum amount of overall deformation of the part.

There exist forging methods of the kind described in the introduction in which some minimum amount of deformation is imposed on the blank during the forging operation C of the final forging step in the beta domain that is performed at the temperature T₂. Thus, in certain applications, the Applicant applies a deformation ratio greater than 0.7 to all points of the part during the forging operation, i.e. each point of the part after the final forging operation in the beta domain has been subjected to a deformation ratio greater than 0.7.

This minimum amount of local deformation imposed during the final forging step in the beta domain makes it possible to obtain a fine microstructure made up of grains that used to be beta grains.

Under such circumstances, and in spite of the fact that the part is at a temperature higher than the beta transus temperature T_(β), the Applicant has found that the final forging step does not enable a fine and uniform microstructure to be produced regardless of the local deformation ratio that is achieved, particularly if the blank (or billet) previously presents a microstructure that is non-uniform, in particular a microstructure with isolated large grains.

In the invention, it can be understood in surprising manner that in spite of the fact that the forging operation during which a minimum deformation ratio is imposed of all points of the billet takes place at a temperature that is lower than beta transus temperature T_(β), a fine and uniform microstructure is produced in the forging, even if the blank (or billet) presents a microstructure that is non-uniform, in particular a microstructure with isolated large grains.

This solution also presents the additional advantage of further making it possible to avoid modifying the conditions in which the final forging step is performed, which step is relatively difficult to implement because of the temperature reached (temperature T₂>beta transus temperature T_(β)).

A minimum deformation ratio is provided at all points of the billet as a result of the forging operation proper in the blank forging step, which ratio is at least 0.2, and said minimum deformation ratio is preferably 0.3 and better 0.4.

In practice, it is verified that the minimum local deformation ratio has indeed been achieved at all points in the billet by using computer tools for numerical simulation of the forging operation proper.

Thus, by using such computer tools, it is possible to ensure that the criteria for some minimum amount of deformation are satisfied.

Preferably, the method relates to a titanium alloy of the alpha-beta type.

In particular, it is preferable to use one of the following two alloys:

-   -   the titanium alloy known as “Ti 6242” or Ti-6Al-2Sn-4Zr-2Mo         which comprises about 6% aluminum, 2% tin, 4% zirconium, and 2%         molybdenum (the TA6Zr4DE alloy in metallurgical nomenclature);         or     -   the titanium alloy known as “Ti 17” or TACD4 or         Ti-5Al-4Mo-4Cr-2Sn-2Zr which has about 5% aluminum, 4%         molybdenum, 4% chromium, 2% tin, and 2% zirconium.

FIGS. 2 and 3 show respective photographs of microstructures corresponding to the situation before performing the forging method of the invention, and the modified microstructure that results from the forging method of the invention.

Thus, in FIG. 2, there can be seen a very large grain of non-recrystallized beta phase, with a size of the order of 20 mm×8 mm, as observed in billets.

In this example, it is titanium Ti17 alloy and a forging method has been performed comprising a single blank forging step in which, for this blank forging step, said forging operation achieves deformation greater than a minimum deformation ratio equal to 0.3 at all points in the billet.

The result visible in FIG. 3 shows that the very large beta phase grain has indeed recrystallized since it presents a microstructure that is uniform and fine, i.e. with a grain size of the order of 50 μm to 100 μm.

In general, by means of the forging method of the present invention, the resulting thermomechanical part is a forging of beta-forged alpha/beta alloy that presents a microstructure that is finer or more refined than the microstructure of the starting billet, the fine microstructure that is obtained presenting a typical grain size of the order of a few hundreds of micrometers at most.

Amongst the other possible variants of the forging method of the invention, there are provided:

-   -   a forging method comprising at least two blank forging steps         while ensuring that for at least one of the two successive blank         forging steps said forging operation implements, at all points         of said billet, deformation greater than a minimum deformation         ratio equal to 0.2; or     -   a forging method comprising at least first and second blank         forging steps and in which, for one of the first and second         blank forging steps, said forging operation implements, at all         points of said billet, deformation that is greater than a         minimum deformation ratio equal to 0.3; or     -   a forging method including at least two blank forging steps and         wherein, for each blank forging step, said forging operation         implements, at all points in said billet, deformation that is         greater than a minimum deformation ratio equal to 0.2.

Under such circumstances, it is possible to provide for two, three, four, or more blank forging steps. 

1-16. (canceled)
 17. A method of forging a thermomechanical part of beta or alpha/beta titanium alloy, the method comprising: providing a billet made from a titanium alloy possessing a beta transus temperature T_(β); performing at least one blank forging operation on the billet, in which the billet is heated to a temperature T₁ lower than the beta transus temperature T_(β) prior to performing the forging operation proper during which the billet is subjected to plastic deformation, thereby obtaining a blank, and then allowing the blank to cool; and performing a final forging operation on the blank, in which the blank is heated to a temperature T₂ higher than the beta transus temperature T_(β) prior to performing the forging operation proper during which the blank is subjected to plastic deformation, thereby obtaining a forging, and then cooling the forging; wherein the forging operation of the blank forging implements, at all points of the billet, local deformation that is greater than a minimum deformation ratio.
 18. A forging method according to claim 17, wherein the minimum deformation ratio is not less than 0.2.
 19. A forging method according to claim 17, wherein the minimum deformation ratio is 0.3.
 20. A forging method according to claim 17, wherein the minimum deformation ratio is 0.4.
 21. A forging method according to claim 17, comprising at least first and second blank forging operations, and wherein for the first or the second blank forging operations, the forging operation implements, at all points in the billet, deformation that is greater than a minimum deformation ratio of 0.3.
 22. A forging method according to claim 17, comprising a single blank forging in which, during the blank forging, the forging operation implements, at all points of the billet, deformation that is greater than a minimum deformation ratio of 0.3.
 23. A forging method according to claim 17, comprising at least two blank forging operations, and wherein for the at least two successive blank forging operations, the forging operation implements, at all points of the billet, deformation that is greater than a minimum deformation ratio of 0.2.
 24. A forging method according to claim 17, comprising at least two blank forging operations, and wherein for each blank forging operation, the forging operation implements, at all points of the billet, deformation that is greater than a minimum deformation ratio of 0.2.
 25. A forging method according to claim 17, wherein the titanium alloy is an alpha/beta type alloy.
 26. A forging method according to claim 17, wherein the titanium alloy is “Ti 6242” or Ti-6Al-2Sn-4Zr-2Mo.
 27. A forging method according to claim 17, wherein the titanium alloy is “Ti 17” or Ti-5Al-4Mo-4Cr-2Sn-2Zr.
 28. A method of fabricating a thermomechanical part made of titanium alloy, the fabrication method comprising a forging method according to claim
 17. 29. A thermomechanical part made of titanium alloy in which the fabrication method includes the forging method according to claim 17, the thermomechanical part being a forging of beta-forged alpha/beta alloy presenting a microstructure that is fine and uniform with a grain size of an order of 50 μm to 100 μm.
 30. A thermomechanical part according to claim 29, forming a rotary part of a turbomachine.
 31. A thermomechanical part according to claim 29, forming a high-pressure compressor disk.
 32. A turbomachine comprising a thermomechanical part according to claim
 29. 